You can use cryptography to perform parts of verification. Specifically, you can use anonymous credentials to prove to a specific website that you have a credential. But that credential must then be issued by some authority that actually examines your physical "proof of age" (driver's license etc.) and issues it to your device.

The problem is that, at least at the moment, simply obtaining a credential like this has a huge chilling effect. You need to mail stuff into some crummy company chosen by the government, which then processes it and takes time. That company might be breached, which identifies you as "a person who is specifically interested in accessing adult content." Many of these problems can theoretically be fixed, but fixing them would have to be the goal of the authorities -- and I suspect it is not.